Interference Engine
Basically, some engines are built to a close tolerance where the piston, at the top of the stroke when the valves are closed, is occupying the same space that the valves will be occupying when they are open and the piston is at the bottom of the stroke.
This is fine as long as normal timing is being kept - the piston rises as the valves close, then it decends as they open, so they never make contact.
BUT...snap a timing belt on this type of motor and...the valves stop opening and closing, but those pistons keep slamming up and down....bent valves, broken lifters, even a broken camshaft could be the outcome....